Anchoring devices are commonly used in oil and gas wellbores to anchor down-hole tools—such as packers or bridge plugs—to a string of casing that lines the wellbore. Many such tools require anchoring devices that are able to resist axial movement with respect to the wellbore when an axial load is applied.
The most common type of anchor device is the slip and cone assembly. The cone is comprised of a tube or bar with a cone shaped outer surface (or flats, or angles milled at an angle with respect to the cone's longitudinal axis). The slip is designed with a gripping profile on its exterior surface to engage the inner diameter of the casing, and has a conical (or tapered flat, or angled) surface on its interior that is designed to mate with the cone.
While existing slip and cone assemblies have generally proven to be reliable anchoring devices, characteristics of conventional slip and cone assemblies limit their versatility in actual down-hole environments. For example, conventional slip and cone arrangements transfer load by changing the axial force applied into a combination of axial and radial forces that are transmitted into the casing. The percentage of axial and radial forces applied is dependent upon cone angle and slip-to-cone friction; when high axial loads are applied, the radial force component can exceed the hoop strength of the casing, consequently damaging the casing. Furthermore, the cone may collapse inward below its original diameter and impede function of the down-hole tool (or restrict the passage of items or fluid through the bore). Thus, there is a need in the art for an anchor device that does not damage the casing and can resist cone collapse when subjected to radial force.
Second, the wellbores that down-hole tools are used in are commonly lined with casing that is manufactured to A.P.I. specifications. Such casing is typically specified by: (1) a nominal outer diameter dimension, and; (2) a specific weight-per-foot. The inner diameter can vary between a minimum dimension (known as “drift diameter”) and a maximum dimension controlled by a maximum tolerance outer diameter and a minimum weight-per-foot. Thus the inner diameter range of a particular size and weight of casing made to A.P.I. specifications can be quite large. In addition, for each nominal size of casing, there are several weights available. Conventional slip and cone assemblies rely on the cone being smaller than the drift diameter of the heaviest weight casing it can be run in. The slip also starts out at a diameter less than the drift diameter of the heaviest weight casing. Therefore, current slip and cone assemblies are limited in maximum casing range to casing inner diameters that are less than the cone diameter plus twice the slip thickness. Otherwise, the slip would pass axially over the cone, and the anchor would be unable to transfer any load. Thus, for reasons of simplicity and inventory reduction, there is a need in the art for an anchoring device that covers as wide a range of casing inner diameters as possible.
Third, as the slip rides up the cone, the contact area between the slip and cone becomes smaller and smaller, until the outer surface of the slip engages the inner diameter of the casing. As the contact area between the slip and cone becomes smaller, the ability of the cone to support the slip is diminished, and consequently so is the casing area that the radial forces have to act on (which increases the stress in the casing). As the casing inner diameter increases due to strain from the applied load, a continued reduction in the supported cone/slip contact occurs, and the anchoring capacity decreases, until, finally, the casing fails, the slip overrides the cone, or the cone collapses. Thus, there is a need in the art for an anchoring device whose performance is not compromised when the inner diameter of the casing is increased by slip-induced radial forces, or when it is used in lighter weights of casing with larger inner diameters.
Fourth, conventional slips start out with an outer gripping surface manufactured to a certain diameter. As the slip is moved up the cone, it contacts the inner diameter of the casing. The inner diameter of the casing will fall within a range of diameters—only one of which will match the outer diameter of the slip. A mismatch in curvature will cause the slip to contact the casing at points, rather than contact it uniformly over the slip/casing surface. With slips and cones that have mating conical surfaces, a similar curvature mismatch will occur between the inner diameter of the slip and the cone as the slip rides up. This type of mismatch usually leads to deformation of the slip at higher loads, and the stress concentrations induced by the point loading can damage the casing, as well as the slip and/or cone. Thus, there is a need in the art for a slip with a variable outer diameter that is capable of limiting or eliminating curvature mismatch with a range of casing inner diameters, as well as with the cone.
Fifth, the cone angle of a slip and cone anchor is always a compromise between having an angle that is shallow enough to allow the anchor to grip the casing, yet steep enough to limit the radial forces transmitted to the casing and cone. Thus, there is a need in the art for an anchor device that exerts sufficient radial force to ensure engagement with the casing, yet limits that radial force below a magnitude that would damage the casing or cone.
Sixth, one of the most common methods for increasing the load capacity of a slip and cone assembly is to increase the area that the radial forces are distributed across. This can be done by either increasing the lengths of the slip and the cone, or by increasing the numbers of slips and cones used. However, increasing the slip length or number adds to the cost and length of the down-hole tool. Thus, there is a need in the art for a high-load anchor device that has fewer slips and is shorter in length than current devices.
Seventh, when down-hole tools are run in wellbores that are deviated or horizontal, the tool string lays to the low side of the wellbore. When a conventional slip and cone assembly is deployed, part of the force to set the anchor is consumed trying to lift the tool string so that it is centered in the wellbore. If the setting force of the anchor is limited, there may not be sufficient force to center the tool string, and the low side of the slip will contact the low side of the casing, which often collects debris. With the only slip contact area of the casing covered with debris, the ability of the slip to initiate a grip is reduced, increasing the likelihood that it will slide in the casing. Thus, there is a need in the art for an anchor device whose performance is unaffected by the presence of debris on the low side of a non-vertical wellbore.
Eighth, in wellbore anchoring applications such as liner hangers, bypass area around the slips is necessary to circulate fluids and cement through the casing. Current liner hangers create bypass areas by using several slips and cones with gaps between them. However, current slip and cone designs close off the area above the cone as the slip travels up to grip the casing, reducing bypass area. Using few slips with large gaps between them causes the casing and cone to be radially point loaded in a way that induces a non-round section, increasing stresses and impeding the passage of tools through the effective reduced inner diameter. Adding more slips maintains the circular shape of the casing, but adds to cost and complexity. Thus, there is a need in the art for an anchor device that radially loads the casing and cone in a more uniform manner and maintains a large bypass area even after the slips have initiated a grip with the casing.
Ninth, in expandable liner applications, current practice is to stay tied onto the liner during cementing and expansion, and then set a liner hanger during or after the expansion process. This method increases the risks associated with not being able to activate the liner hanger and/or release the running tool when cement is displaced around the liner top. Conventional slip and cone assemblies are not conducive to expansion of the liner hanger after the anchors have been set because of the close proximity of the mandrel, cone, and slip. Thus, there is a need in the art for a liner hanger than can be run with expandable liners and set prior to the liner or liner hanger expansion.
Therefore, a need exists in the art for an improved slip and cone assembly. The above concerns are addressed by the assembly of the present invention.